A wheel of a vehicle and a brake rotor for constituting a disc brake which serves as a brake are rotatably supported by a knuckle which constitutes a suspension system. That is, an outer race serving as a stationary race and constituting a bearing unit for a wheel is fixed in a circular support hole portion formed on this knuckle by means of a plurality of bolts. On the other hand, the wheel and the rotor are connected and secured to a hub for constituting a bearing unit for a wheel by means of a plurality of studs and nuts.
Plurality outer race tracks respectively serving as stationary race tracks are formed on the inner peripheral surface of the outer race 6, while a coupling flange is formed on the outer peripheral surface thereof. Such an outer race is fixed to the knuckle by coupling this coupling flange to the knuckle with a bolt.
On the other hand, an attachment flange is formed in a part of the outer peripheral surface of the hub which is protruded from an outer end opening of the outer race (“outer with respect to the axial direction” means the outer side in the width direction in a state of being assembled in the vehicle, which is the same throughout this specification). The wheel and the rotor are connected and fixed to the outer side surface of this attachment flange with the studs and nuts. Also, an inner race track is formed on the outer peripheral surface of the middle portion of the hub at a part opposite to the outermost race track out of the plurality of outer race tracks. Further, an inner race is fitted and fixed onto a small step portion which is formed in an inner end portion of the hub. Another inner race track formed on the outer peripheral surface of the inner race is arranged to face the innermost outer race track, out of the plurality of outer race tracks.
Plurality of balls are provided to be rotatable between each of the outer race tracks and inner race tracks in a state that they are respectively held by retainers. With this structure, a double row angular type ball bearing which is a back-to-back duplex bearing is constituted, and the hub is supported to be rotatable on the inner side of the outer race to be capable of supporting a radial load and a thrust load. Seal rings are provided between each of the inner peripheral surfaces of the both end portions of the outer race and the outer peripheral surface of the middle portion of the hub, and the outer peripheral surface of the inner end portion of the inner race, respectively, so as to block a space in which the balls are provided from the outer space.
When the bearing unit for a wheel is used, the outer race is fixed to the knuckle and, at the same time, the wheel and the rotor to which a tire is combined are fixed to the attachment flange of the hub. The rotor and a support and a caliper fixed to the knuckle are combined with each other to constitute the disc brake. At braking, a pair of pads which are provided to sandwich the rotor therebetween are pressed against the both sides of this rotor.
Incidentally, it is known that, at the time of braking of the vehicle, there is often generated a vibration followed by unpleasant noise called a judder. It is also known that such a vibration is caused by various reasons including an uneven frictional condition between a side surface of the rotor and a lining of the pad. However, a shake of the rotor is known as a great cause of the vibration. That is, a side surface of the rotor should make a right angle with the center of rotation of this rotor properly. However, it is difficult to obtain a perfect right angle in this case due to an inevitably error in manufacturing. As a result, it is unavoidable that the side surface of the rotor shakes slightly in the direction of the rotary shaft during a running of the vehicle. If such a shake becomes great, a judder is generated when linings of the pair of pads are pressed against the both sides of the rotor for braking. Apart from this judder generation, there are several problems including that a surface against which the linings of the pads of the rotor are pressed is partially abraded or a vibration of the vehicle body becomes great at the time of braking due to a shake of the rotor.
In order to solve the problems such as generation of a judder, it is important to suppress a shake of the rotor with respect to the axial direction (axial shake). In order to suppress this shake, it is required to improve the degree of perpendicularity of an attachment surface of the attachment flange with respect to the center of rotation of the hub and to improve the planar precision of this attachment surface itself. Particularly, as to the degree of perpendicularity, it is important to improve the precision in the positions and the configurations of the attachment surface and the race track surfaces (the outer race tracks and the inner race track). Technologies for improving the precision are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 10-217001 and 2000-234624.
Meanwhile, a shake of the rotor may be caused when a mutual difference between the diameters of the balls assembled in the bearing unit for a wheel (the difference between the average diameter of the greatest ball and that of the smallest ball, out of the balls to be assembled in the bearing unit) is great. That is, if a difference between the diameter of some ball present in a ball row and that of other balls present in the same ball row is great, the hub is rotated in a shaking manner with respect to the outer race upon revolution of these balls. For example, if the diameter of a certain ball in the inner ball row and that in the outer ball row, the rows constituting the bearing unit for a wheel are greater than the diameter of the other balls in the same ball rows, a distance between the inner peripheral surface of the outer race and the hub or the outer peripheral surface of the inner race becomes greater in a part in which certain balls having the greater diameter are present. For this reason, the central axis of the hub is inclined in the counterclockwise direction with respect to the central axis of the outer race. As a result, when the bearing unit is operated, the hub shakes in a rotating manner with respect to the outer race and the rotor which is fixed to the attachment flange formed on the outer peripheral surface of this hub shakes in the direction of the rotary shaft. Then, the greater the difference between the diameter of the certain balls and that of other balls is, the more striking the scale of this shake is.
Recently, in order to prevent the generation of a judder at braking, it is demanded to sufficiently suppress a shake of the rotor. However, if the mutual difference in lot diameter between the balls assembled in the bearing unit for a wheel is great, it is difficult to satisfy such a demand as described above. Accordingly, it is required to reduce the mutual difference in diameter between the balls in order to sufficiently suppress a shake of the rotor.